Fix Actor Collision Checks Again

* Recent collision update caused a regression where the player would get
  "stuck" while standing on top of a solid doodad, unable to walk left
  or right.
* When deciding if the actor is on top of a doodad, use the doodad's
  Hitbox (if available) instead of the bounding box. This fixes the
  upside-down trapdoor acting solid when landed on from the top, since
  its Hitbox Y coordinate is not the same as the top of its sprite.
* Cheats: when using the noclip cheat in Play Mode, you can hold down
  the Shift key while moving to only move one pixel at a time.

dev-assets/doodads/crumbly-floor/crumbly-floor.js

@@ -19,10 +19,6 @@ function main() {
 	var startedAnimation = false;
 
 	Events.OnCollide(function(e) {
-		// Only trigger for mobile characters.
-		if (!e.Actor.IsMobile()) {
-			return;
-		}
 
 		// If the floor is falling, the player passes right thru.
 		if (state === stateFalling || state === stateFallen) {
@@ -36,6 +32,11 @@ function main() {
 				return false;
 			}
 
+			// If movement is not settled, be solid.
+			if (!e.Settled) {
+				return false;
+			}
+
 			// Begin the animation sequence if we're in the solid state.
 			if (state === stateSolid) {
 				state = stateShaking;

docs/Shell.md

@@ -9,6 +9,8 @@
 * `import antigravity` - during Play Mode, disables gravity for the player
   character and allows free movement in all directions with the arrow keys.
   Enter the cheat again to restore gravity to normal.
+  * Note: under antigravity, hold down the Shift key to lower the player
+    speed to only one pixel per tick.
 * `ghost mode` - during Play Mode, toggles noclip for the player character.
 
 ## Bool Props

pkg/play_scene.go

@@ -427,6 +427,12 @@ func (s *PlayScene) Draw(d *Doodle) error {
 func (s *PlayScene) movePlayer(ev *event.State) {
 	var playerSpeed = balance.PlayerMaxVelocity
 
+	// If antigravity enabled and the Shift key is pressed down, move the
+	// player by only one pixel per tick.
+	if s.antigravity && ev.Shift {
+		playerSpeed = 1
+	}
+
 	var velocity render.Point
 
 	if ev.Left {

pkg/uix/actor_collision.go

@@ -127,6 +127,19 @@ func (w *Canvas) loopActorCollision() error {
 				}
 			)
 
+			// HACK: below, when we determine the moving actor is "onTop" of
+			// the doodad's solid hitbox, we lockY their movement so they don't
+			// fall down further; but sometimes there's an off-by-one error if
+			// the actor fell a distance before landing, and so the final
+			// Settled collision check doesn't fire (i.e. if they fell onto a
+			// Crumbly Floor which should begin shaking when walked on).
+			//
+			// When we decide they're onTop, record the Y position, and then
+			// use it for collision-check purposes but DON'T physically move
+			// the character by it (moving the character may clip them thru
+			// other solid hitboxes like the upside-down trapdoor)
+			var onTopY int
+
 			// Firstly we want to make sure B isn't able to clip through A's
 			// solid hitbox if A protests the movement. Trace a vector from
 			// B's original position to their current one and ping A's
@@ -147,10 +160,16 @@ func (w *Canvas) loopActorCollision() error {
 				// Touching the solid actor from the side is already fine.
 				var onTop = false
 
+				var (
+					lockX int
+					lockY int
+				)
+
 				for point := range render.IterLine(
 					origPoint,
 					b.Position(),
 				) {
+					point := point
 					test := render.Rect{
 						X: point.X,
 						Y: point.Y,
@@ -158,11 +177,6 @@ func (w *Canvas) loopActorCollision() error {
 						H: rect.H,
 					}
 
-					var (
-						lockX bool
-						lockY bool
-					)
-
 					if info, err := collision.CompareBoxes(boxes[tuple.A], test); err == nil {
 						// B is overlapping A's box, call its OnCollide handler
 						// with Settled=false and see if it protests the overlap.
@@ -176,39 +190,50 @@ func (w *Canvas) loopActorCollision() error {
 						// Did A protest?
 						if err == scripting.ErrReturnFalse {
 							// Are they on top?
-							if render.AbsInt(lastGoodBox.Y+lastGoodBox.H-boxes[tuple.A].Y) <= 2 {
+							aHitbox := doodads.GetBoundingRectHitbox(a, a.Hitbox())
+							if render.AbsInt(test.Y+test.H-aHitbox.Y) == 0 {
 								onTop = true
+								onTopY = test.Y
 							}
 
 							// What direction were we moving?
 							if test.Y != lastGoodBox.Y {
-								lockY = true
-								b.SetGrounded(true)
+								if lockY == 0 {
+									lockY = lastGoodBox.Y
+								}
+								if onTop {
+									b.SetGrounded(true)
+								}
 							}
 							if test.X != lastGoodBox.X {
 								if !onTop {
-									lockX = true
+									lockX = lastGoodBox.X
 								}
 							}
 
-							// Move them back to the last good box, locking the
-							// axis they were moving from being able to enter
-							// this box.
-							tmp := lastGoodBox
+							// Move them back to the last good box.
 							lastGoodBox = test
-							if lockY {
-								lastGoodBox.Y = tmp.Y
-							}
-							if lockX {
-								lastGoodBox.X = tmp.X
-								break
+							if lockX != 0 {
+								lastGoodBox.X = lockX
 							}
 						} else {
+							// Move them back to the last good box.
 							lastGoodBox = test
 						}
+					} else {
+						// No collision between boxes, increment the lastGoodBox
+						lastGoodBox = test
 					}
 				}
 
+				// Did we lock their X or Y coordinate from moving further?
+				if lockY != 0 {
+					lastGoodBox.Y = lockY
+				}
+				if lockX != 0 {
+					lastGoodBox.X = lockX
+				}
+
 				if !b.noclip {
 					b.MoveTo(lastGoodBox.Point())
 				}
@@ -220,6 +245,10 @@ func (w *Canvas) loopActorCollision() error {
 				)
 			}
 
+			if onTopY != 0 && lastGoodBox.Y-onTopY <= 1 {
+				lastGoodBox.Y = onTopY
+			}
+
 			// Movement has been settled. Check if B's point is still invading
 			// A's box and call its OnCollide handler one last time in
 			// Settled=true mode so it can run its actions.